Because a magnetic recording thin film medium has a high packing density of magnetic substance, is suitable for high-density recording, and is profitable for the electro-magnetic transformation characteristics as compared with a magnetic recording particulate medium, the magnetic recording thin film medium has been recently practically used.
In a magnetic recording thin film medium, usually a metal magnetic layer is formed on a non-magnetic support and further a lubricant layer is formed on the metal magnetic layer, whereby the still characteristics, the friction characteristics, etc., are improved.
However, because in the magnetic recording thin film medium, a magnetic layer is not formed by coating a coating composition formed by dispersing a ferromagnetic powder in a binder on a support, followed by drying as in magnetic recording particulate media, but the metal magnetic layer is formed on a support by a vacuum vapor deposition method, a sputtering method, an ion plating method, etc., by forming a lubricant layer only on the metal magnetic layer, there are problems that the magnetic recording medium is lacking in the reliability thereof, such as the friction characteristics, the still characteristics, the corrosion resistance, and the like.
Accordingly, recently, it is proposed to solve the above-described problems by forming a protective layer of a plasma polymerized film between the magnetic layer and the lubricant layer as disclosed in Japanese Patent Publication (KOKOKU) No. 7-66528.
However, in this case, the weather resistance, the practical running test, etc., can be improved, but because the adhesive property of the protective layer and the lubricant layer is inferior to cause releasing of the lubricant, there is a problem that attaching of the released lubricant to a head (head attaching), clogging of a head with the released lubricant (head clogging), etc., occur, whereby the magnetic recording medium is lacking in the durability.
Thus, as one of the methods of improving the adhesive property between the protective layer and the lubricant layer, a method of forming a functional group at the surface of the protective layer by surface-treating the protective layer and coating thereon a lubricant having an OH group or a COOH group at the terminal of the lubricant capable of chemically modifying the functional group is proposed as disclosed in Japanese Patent Public Disclosure (KOKAI) No. 63-136316. Furthermore, it is reported to improve the durability of a magnetic recording medium by forming a carbonyl group at the surface of the protective layer as described in Japanese Patent Public Disclosure (KOKAI) No. 3-35419, etc.
Also, Japanese Patent Publication (KOKOKU) No. 8-33994 proposes a technique of improving of the adhesive property between the protective layer and the lubricant layer by forming a diamond-like carbon layer on a metal magnetic thin film as a protective layer, immediately thereafter forming an oxide layer on the above-described diamond-like carbon layer by a vacuum treatment method, and thereafter forming thereon a lubricant layer.
However, even by these techniques described in above-described Japanese Patent Public Disclosure (KOKAI) Nos. 63-136316 and 3-35419, the effects of improving the sliding resisting lubricative property in the contact-type continuous sliding between a head and the surface of the magnetic recording medium, the running durability, the friction characteristics, etc., have not yet been obtained to the sufficiently satisfactory extent.
Also, in the method disclosed in above-described Japanese Patent Publication (KOKOKU) No. 8-33994, although an effect is obtained in the reduction of the head clogging, etc., there is a problem that a sufficient effect is not obtained in the still durability. The reason is perhaps considered to be that in the method disclosed in Japanese Patent Publication (KOKOKU) No. 8-33994, because a bias voltage is applied between a support conveying roller and the metal magnetic layer at the formation of the protective layer and the oxide layer, the damage of the protective layer at the formation of the oxide layer becomes large. This is considered that when a bias voltage is applied, a definite amount of the energy of the bias voltage is always additionally supplied to the ions generated by plasma discharging to the protective layer at the formation of the oxide layer, and even when the plasma discharging power at the initiation of the plasma treatment is low, the bias voltage-applied power is added to the plasma discharging power, whereby the damage given to the surface of the protective layer in the formation of the oxide layer through the plasma treatment process becomes large, which results in lowering the characteristics of the whole plasma polymerized film to lower the still characteristics.